The invention is generally related to systems for curing thermosetting polymeric materials and more particularly to apparatus and methods for curing resin-matrix composites. The curing of a thermosetting polymer composite, such as a precatalyzed, preimpregnated fibrous material (prepreg) typically involves: (1) temperature control to initiate and sustain polymerization and obtain the release of volatiles; (2) pressure control to assure proper compaction of the laminate, removal of voids and release of excess resin; and (3) vacuum control to enhance the removal of volatile materials. The proper control of the heat-up rate, the temperature gradient in the part, and the exact time at which pressure to consolidate the composite is applied are critical factors. Optimum mechanical properties in a cured composite are obtained when it is substantially void-free and has a proper resin/fiber ratio, i.e., in a preselected narrow processing range.
In the past, the curing of resin matrix composites has generally followed a fixed cure schedule or cycle, with the amount and rate of heat application and pressure application being predetermined by the method of using resin sampling techniques or small test specimens to determine the degree of curing and onset of gelation of the resin which takes place during a given cure cycle. The information gathered from the sampling process was then extrapolated to curing other like composites. Such sampling sometimes included the monitoring of the dielectric constant and dissipation (pseudo-viscosity) to aid in formulating a fixed cycle for applying pressure and vacuum to the composite during the pressure application window which occurs at the onset of gelation, as described in "In-Process Controlled Curing of Resin Matrix Composites", Michael J. Yokata, 22nd National SAMPE Symposium and Exhibition, Volume 22.
However, there are many inherent limitations in the present curing methods. For example, processes which are successful with small test panels often fail when extrapolated to full-size articles. Further, batch variations in the prepreg, slow polymerization (advancement) in storage, the presence of exothermic reactions, and variable processing equipment characteristics present unpredictable variables. In addition, cure requirements will change with the thickness of the composite and the degree of B-staging of the resin. Also, different schedules must be established for composites having differing resin systems or differing concentrations in a given resin system.
The prior art includes various attempts to obtain measurements of dielectric constants and other characteristics for use in controlling resin cures. The patent to Baum, et al, U.S. Pat. No. 3,846,073, discloses a resin curing apparatus wherein dielectric constant, pressure and temperature are measured for the purpose of controlling the polymer production. Temperature and pressure are adjusted in response to the developed data, but the process is not automated. The patent to Garst, U.S. Pat. No. 3,985,712, discloses an apparatus for the automatic control of resin curing. Batch temperature is periodically read and compared with previously empirically determined temperature-time characteristics which are stored in a memory. An apparatus for programmed dielectric constant testing is disclosed in the patent to Schmidt, U.S. Pat. No. 4,023,096. A capacitive sensor is revealed in the patent to Shawhan, U.S. Pat. No. 2,765,219. The sensor has a memory system which compares values of a variable during successive time intervals and provides a control depending on the nature of the variable.
None of the above prior art systems is satisfactory for achieving a completely cured resin composite having optimum mechanical properties. Accordingly, there is a need for a simple, efficient, reproduceable and accurate method for determining optimum times for applying temperature and pressure to a prepreg to insure optimum end results. Such method should preferably be automatic, have a flexibility capable of handling prepregs with different given characteristics, complete the curing process in the shortest feasible amount of time, and render a cured composite which is both substantially free of voids and which has a minimal resin content lying in a given acceptable range.